Ford Mondeo Diesel - Smokey and engine hunts?

I have a 54 plate Mondeo 130ps with a 6 speed box. Just lately it has started to become very smokey. Thick black unburnt diesel and not just the little bits under load as they normally do but enough to block out a car 50 metres behind! Also, when the accelerator is held at a constant speed or on cruise control, the vehicle starts to hunt. It seems to cut out for a fraction of a second and then pick up. This could last for 20 seconds or so then it goes normal for 20 seconds then it starts again.
The car starts first time every time without fail and ticks over smoothly.
Any ideas?
Steve